not sure what kind, seems to be acacia/tree type that should have yellow flowers. Purchased about 2ft high. Care of mimosa

Have a mimosa now about 3 years old & kept in cold greenhouse. This has never flowered and is now touching top of greenhouse - approx. 7ft - more like a small tree! Can I prune it? when? and how can I encourage it to flower? Live in North Cumbria - cold & wet!! thanks